A conventional optical fiber connecting device will be explained (see, e.g. Japanese Unexamined Patent Application Publication (JP-A) No. 2000-19355 (page 3, third paragraph, line 45 to page 4, fifth paragraph, line 15, FIGS. 1-4 and 7-9)).
FIG. 10 is a perspective view of an optical fiber connecting device before connection, in which two optical connectors A and B are connected to each other through an adapter C (shown in a broken fashion). The optical fiber connecting device comprises the first optical connector A, the second optical connector B, the adapter C, two lock springs 61, 61 for locking the first optical connector A and the second optical connector B to the adapter C, respectively, and so on.
As shown in FIGS. 8 and 9, the first optical connector A and the second optical connector B each comprise tape fibers 51 each formed by integrally fixing many optical fibers 52, arrayed side by side, into a flat plate shape by the use of a synthetic resin or the like, clamp members 53 each clamping the many optical fibers 52 that are exposed by removing a cover of the tape fiber 51 at its end portion, an alignment member 54 holding the tip end portions of the respective optical fibers 52 in the state where those tip end portions are aligned, a casing member 55 holding the clamp members 53, the alignment member 54, and so on, and a first fitting piece 56a or a second fitting piece 56b detachably held in a recess portion 55a of the casing member 55. The alignment member 54 is slidable inside the casing member 55 in an axial direction of the optical fibers 52. An elastic member 56b1 is provided on the lower side (the side pressing the optical fibers 52) of the second fitting piece 56b. 
As the fitting piece, either one of the first fitting piece 56a shown in FIG. 9, (A) and the second fitting piece 56b shown in FIG. 9, (B) is selected and used. As will be described later, in the state of being fitted in the recess portion 55a of the casing member 55, the first fitting piece 56a is located on the inner side of the casing member 55 and does not press portions of the optical fibers 52 located between the clamp members 53 and the alignment member 54. On the other hand, in the same state, the second fitting piece 56b is located on the inner side of the casing member 55 and presses portions of the optical fibers 52 located between the clamp members 53 and the alignment member 54 as shown in FIG. 11 so as to substantially linearly hold the respective optical fibers 52 at these portions.
The adapter C comprises an alignment member 57 and a casing member 58 holding the alignment member 57. The alignment member 57 comprises, on its surface, micro-sleeves 57a into which the optical fibers 52 are inserted, respectively, a V-groove base plate 57b having V-grooves formed parallel to each other and holding the micro-sleeves 57a, respectively, a holding plate 57c holding the V-groove base plate 57b, and so on, thereby holding the tip end portions of the optical fibers 52 in a passable manner. Since it is configured such that the first fitting piece 56a or the second fitting piece 56b is detachably held in the recess portion 55a of the casing member 55 in the first optical connector A or the second optical connector B, bending for obtaining a buckling load (a load that produces a contact force between tip end surfaces of a pair of optical fibers by bending of at least one of the optical fibers) can be produced at each of the optical fibers 52 of the first optical connector A or the second optical connector B when assembling the optical connectors.
In the state shown in FIG. 12, by using the first fitting pieces 56a in both optical connectors A and B, the respective optical fibers 52 of both optical connectors A and B can be bent. Therefore, a buckling load is produced at each of the optical fibers 52 of both optical connectors A and B so that the end surfaces of the respective optical fibers 52 of both optical connectors A and B are in pressure contact with each other.
On the other hand, as shown in FIG. 13, by using the second fitting piece 56b in the left optical connector A and the first fitting piece 56a in the right optical connector B, it is possible to bend only the optical fibers 52 of the optical connector B. Therefore, a buckling load is produced at each of the optical fibers 52 of the optical connector B so that the end surfaces of the respective optical fibers 52 of both optical connectors A and B are in pressure contact with each other.